I. Technical Field
The present invention relates to a molten metal holding furnace for supplying a constant quantity of molten metal of nonferrous metal, such as aluminum and aluminum alloys to a casting machine.
II. Description of Related Art
Conventionally, there has been known a molten metal delivering apparatus for supplying a constant quantity of molten metal to a casting machine (see, e.g., JP 3192623 B).
JP 3192623 B discloses a molten metal delivering apparatus including: a melt storage furnace which has, in a hearth face thereof, a melt flow passage opening to be opened and closed by an up/down first cutoff valve; a supply chamber which is provided beside the melt storage furnace and which has a melt flow passage opening in a hearth face thereof and further which is formed so that its internal pressure can be increased and reduced; a fixed molten metal furnace which is provided beside the supply chamber and which has, in a hearth face thereof, a melt flow passage opening to be opened and closed by an up/down second cutoff valve, and further which has, at a side portion thereof, a delivery opening for supplying a constant quantity of molten metal to a casting machine, and a communicating pipe which makes the melt storage furnace, the supply chamber and the fixed molten metal furnace communicated at their respective melt flow passage openings to one another.
For supply of the molten metal in the melt storage furnace into the fixed molten metal furnace, first, the melt flow passage opening of the melt storage furnace is opened, and the melt flow passage opening of the fixed molten metal furnace is closed. In this case, the internal pressure of the supply chamber is reduced, so that the molten metal is supplied from the melt storage furnace to the supply chamber via the communicating pipe. Subsequently, the melt flow passage opening of the melt storage furnace is closed, and the melt flow passage opening of the fixed molten metal furnace is opened. In this case, the internal pressure of the supply chamber is increased, by which the molten metal is supplied from the supply chamber to the fixed molten metal furnace via the communicating pipe.
In the case of the molten metal delivering apparatus described in JP 3192623 A, the communicating pipe is provided for making the melt storage furnace, the supply chamber and the fixed molten metal furnace communicated at their hearths to one another, so that impurities such as oxide contained in the molten metal are more easily deposited within the communicating pipe for structural reasons. Therefore, during long-term operations, it may occur that the communicating pipe is blocked by deposited impurities, obstructing a smooth flow of the molten metal. There is a further problem that the impurities may flow into the fixed molten metal furnace along with the molten metal, making it impossible to ensure a clean molten metal to be supplied to the casting machine. In the case of this molten metal delivering apparatus, since decreases of the molten metal temperature in the communicating pipe and the supply chamber are inevitable, it becomes more difficult to supply molten metal of a constant temperature to the casting machine, thus, involving another problem of controlling the molten metal temperature in the fixed molten metal furnace. Still more, there is a need for a space to be formed above the melt surface in the fixed molten metal furnace, which causes a problem that this space incurs oxidation of the molten metal.
Various holding furnaces for casting use are also conventionally known (see, e.g., JP H11-138250 A, JP 3392544 A).
JP H11-138250 A discloses a casting-use holding furnace which is composed of a holding chamber and a pressurization chamber and which has a cutoff valve for opening and closing a melt flow passage opening located in the holding chamber, the pressurization being divided into a pressurization section for causing a pressurizing gas to apply a pressure onto a top surface of the molten metal, and a melt outlet section for causing the molten metal into a cavity of a metal mold. This casting-use holding furnace has a multilayered lining structure composed of a shell, a heat-insulating layer, a fireproof layer and a melt housing container, as listed from outside toward inside, where the melt housing container is formed into an integral bath as an alumina-base castable refractory.
JP 3392544 B discloses a casting-use holding furnace in which at a valve seat placement portion formed at an opening peripheral portion of the melt flow passage opening on one side closer to the holding chamber, a valve seat formed as a member independent of the above-mentioned holding chamber is provided so that its top surface becomes flush with an inner peripheral surface of the melt housing container, in which arrangement of the melt flow passage opening is opened and closed by bringing a tip of the cutoff valve into or out of contact with the valve seat.
In the case of the casting-use holding furnace described in JP H11-138250 A, since the castable refractory, which forms the integral bath of the melt housing container that makes direct contact with the molten metal has gas permeability, permeation of the molten metal into the castable refractory is unavoidable during repetitions of casting process, so that the permeation causes cracks or damage to occur in the castable refractory. In particular, occurrence of such cracks or damage in the pressurization section or the melt outlet section may obstruct the casting work as a problem. More specifically, as a result of occurrence of cracks or damage in the pressurization section, pressure control that has a direct influence on the casting work becomes unstable, making it impossible to implement stable continuous operation, and in the worst case, resulting in a shutdown of operation. Besides, such cracks or damage may incur leakage of the pressurization gas to the outside, causing accuracy of the pressure control to lower. Meanwhile, with occurrence of cracks or damage in the melt outlet section, whereas the control pressure in the pressurization section is maintained regular, a specified amount of molten metal is not be changed into the cavity of the metal mold, so that the cast article results in a defective product. Besides, since the molten metal sticks more and more on inner wall surfaces of the pressurization section and the melt outlet section, there arises a need for regularly removing sticking matters on the inner wall surfaces. However, because, of fragility of the castable refractory, it is highly likely that the surfaces of the castable refractory may be damaged during the removal work for the sticking matters, as a problem.
In the case of the casting-use holding furnace described in JP 3392544 B, since the top surface of the valve seat and the inner peripheral surface of the melt housing container are flush with each other, impurities generated in the melt holding chamber, especially deposits around the melt flow passage opening may flow into the pressurization chamber, contaminating the molten metal as a problem.